1. Field of the Invention
The present invention relates to methods for raising well masts on offshore drilling platforms or at relatively inaccessible locations on land.
2. Description of the Related Art
Several types of well masts are known which are portable for movement between oil well sites, particularly offshore or remote sites. The portable mast structures typically comprise several smaller and more manageable sections to facilitate transportation. At the oil well location, the individual sections are extended one above the other in a telescoping fashion to make a mast of desired height.
Several types of portable telescoping well mast assemblies are known and are in use at the present time. One known type of telescoping portable mast structure is that of U.S. Pat. No. 4,885,893, commonly owned with the present application, of which Applicants are named as inventors. The mast structure disclosed in this patent provides a method to quickly and efficiently erect a mast structure while reducing damage to the mast sections during raising or erection. It includes a lower section which is raised to a vertical position on a drill floor. An upper section is then pivotally attached at its midpoint in a horizontal position to the top of the lower section, thereafter pivoted to a vertical position and then raised until a lower end of the upper section can be secured to an upper end of the lower section.
Well masts of the type disclosed in Applicants' prior patent have been accepted, particularly in the offshore drilling industry. Applicants have found that there remains room for improvement particularly when the mast has to be higher. For example, an upper section of this type of well mast is relatively large and cumbersome since it is taller than any of the other sections. This upper section also includes at its upper end as integral components both a crown platform section as well as the drilling rig traveling block. Experience has proven that the overall height, width, strength and lifting capacity of the completed well mast structure of this type is essentially limited by the upper section. This is due to the size, weight and other practical load limitations of a crane available on the platform crane used to lift each of the well mast sections during assembly. Although it was suggested that a middle section could also be added between the upper and lower mast sections following a similar procedure and more intermediate sections could be added, this was generally not the case in practice. Due to the limited load bearing capacity and strength of each section, usually at most only an upper, middle and lower section could be included as an entire well mast structure.
The completed mast structure of Applicants' prior patent typically had a clear height of approximately 96 feet, and a net hook capacity of approximately 240,000 pounds. With drilling of ever deeper wells, it would be desirable to provide a taller and stronger well mast structure having a greater lifting capacity. However, conflicting structural considerations imposed design limits. Any attempt to make the upper mast section larger or taller was limited by the lifting capacities of available cranes. Further, since no more sections could effectively be added, the mast structure erected using the procedure found in Applicants' prior patent had, in effect, a self-constrained height and lifting capacity.